A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to herein as a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In some circumstances it may be desirable to prevent irradiation (which may also be referred to as preventing exposure) of an outer region of the substrate. The outer region may for example be a peripheral region (e.g. edge region) of the substrate.
One such circumstance occurs, for example, when “packaging” an IC (i.e. mounting onto a board). It has been conventional to use wires to connect an IC to a board. However, in recent years the distance between locations to which wires are to be bonded has become progressively smaller, and it has been more difficult to use wire bonding. A process which is known as flip-chip bumping is increasingly used to connect ICs to boards instead of using connection wires. In flip-chip bumping, solder (or some other metal) is provided at specific locations on each IC on a substrate. The substrate is inverted and bonded to a board, for instance by heating the solder such that it melts and then allowing it to cool again.
The solder (or other metal) may itself be provided at specific locations by a lithographic process. In such a process the substrate, which may comprise a plurality of ICs, is provided with a layer of radiation-sensitive material (resist). A lithographic apparatus may be used to irradiate the resist and the resist subsequently selectively removed at the specific locations in which a solder “bump” is needed (the skilled person will appreciate that these regions may be either irradiated regions or non-irradiated regions, depending upon whether a positive or negative resist is used). The IC may then undergo an electroplating step to apply the solder to the IC at the specific locations. As the skilled person will appreciate, the process of electroplating requires an electrical connection to be made to the article onto which metal is to be deposited. Accordingly, the electroplating step should utilize a resist free area of the substrate for making an electrical connection.
While it may be sufficient to provide a single resist free point for making such an electrical connection, it can be advantageous to provide a continuous ring of resist free substrate around the outer region of the substrate. Such an arrangement may enable a more reliable electrical connection. Furthermore, a continuous resist free ring around the outer edge of the substrate allows an electroplating bath to be conveniently formed using the resist free region. For example, an upstanding wall may be provided on the resist free region of the substrate, such that the substrate forms the base of the electroplating bath.
The resist free region of the substrate may be provided by chemically removing resist, for example by spraying solvent. When a positive resist is used the exposed portion may be provided by irradiating the outer portion of the resist, for example by means of an edge bead removing tool, prior to the step of selectively removing resist. This process is a more accurate method of removal than chemical spraying. Accordingly, it would be useful to prevent irradiation of an outer region of the resist when using a negative resist, such that the resist in this region will subsequently be removed.
Accordingly, a method and apparatus for mechanically masking a workpiece to form an exposure exclusion region (i.e. a non-irradiated region) has been proposed in U.S. Pat. No. 6,680,774 B1. This document teaches an arrangement in which a mask, in the form of a ring of “opaque” material is placed either immediately above or in direct contact with the upper surface of the workpiece (i.e. the substrate), prior to the workpiece being moved into an exposure location for exposure with radiation. However, the applicants have identified several potential problems with the arrangement of this prior art.
Placement of the mask in U.S. Pat. No. 6,680,774 B1 requires the ring to be transferred above and across the width of the substrate. This action requires a relatively large movement of the mask relative to the substrate which provides a time constraint into the operation, especially as the mask must be placed on the substrate following loading onto a substrate support and subsequently removed before the substrate can be unloaded. Furthermore, as the mask is transferred across the substrate there is always a risk of contamination of the substrate, for example from dust or other particles falling from the mask. Another disadvantage is that if the ring is not accurately placed in proximity to the substrate, the excluded region may not be sufficiently accurately defined, which could lead to an inconsistent edge region. The definition of the excluded region may also be adversely affected by the shadow of the mask and by the defocus area of the mask.
It is, therefore, desirable to provide an alternate method and/or apparatus for preventing irradiation of an edge region of the substrate, which may overcome or mitigate at least one of the disadvantages of the prior art.